Dynamic exchange of myosin molecules between thick filaments.

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Abstract
To examine thick filament assembly and myosin exchange, a fluorescence energy transfer assay has been established. Assembly-competent myosin molecules labeled with the sulfhydryl-specific fluorochromes 5-{2-[(iodoacetyl)-amino]ethyl}aminonaphthalene-1-sulfonic acid (IAEDANS) or 5-iodoacetamidofluorescein (IAF) were prepared. Using IAEDANS-labeled myosin as fluorescence donor and IAF-labeled myosin as acceptor, thick filament formation was followed by the decrease in donor fluorescence at 0.1 M KCl/10 mM potassium phosphate, pH 6.9. The critical concentration of myosin-i.e., that concentration that remained unassembled at equilibrium with fully formed filaments-was 40 nM. In FET and 125I-labeled myosin incorporation assays, extersive exchange of myosin between thick filaments was observed. The presence of a critical concentration and the measurements of extensive exchange suggest a dynamic equilibrium between fully polymerized myosin and a small pool of soluble myosin.